In all the debates swirling around whether the U.S. should launch a unilateral military attack on Syria, as the Obama Administration is now seeking to do, one key point is getting very little attention: it would violate international law.

The fundamental rule of contemporary international law is that states cannot attack other states. The U.N. Charter embodies this rule and makes only two exceptions to it: a state can attack another state if it is authorized to do so by a Security Council resolution, or if the attacking state is acting in genuine self-defense.

Neither of these exceptions applies in the case of the U.S. and Syria. But a third basis for the legality of intervention has been suggested: “Customary international law,” a somewhat mysterious concept that comes down to the idea that, if enough of the international community has over time come to treat certain actions as legal, then those actions eventually become legal, even if states have not (yet) formally bound themselves to that conclusion via treaties. And so an argument is also being put forth that the American government should have the legal right to use force in order to alleviate a humanitarian crisis. This was also the position taken by the British government in a memo released last week. The memo argued that “the doctrine of humanitarian intervention” allows military action without U.N. authorization, if “the international community as a whole” agrees that a humanitarian crisis exists, that “there is no practicable alternative to the use of force if lives are to be saved,” and “the proposed use of force … is the minimum necessary to achieve that end and for no other purpose.”

Leaving aside practical questions, the problem with this argument is that it is trying to establish a new customary norm for which there is no precedent. The closest thing to a precedent is NATO’s Kosovo bombing campaign in 1998, but that was based not on humanitarian intervention but on the security interests of NATO members. Even if one ignores that problem, a history consisting of one arguable precedent can hardly be claimed to constitute the customary practice of states.

Indeed, the legal argument for attacking Syria on humanitarian grounds comes down to the claim that, by doing so, the U.S. will be helping to bring about a new customary norm of international law that does not yet exist. The paradoxes involved in this argument are considerable, as it requires asserting that we are violating international law to punish Syria for violating international law, but we are doing so in such a way so as to — eventually — change international law so that our violation won’t be a violation in the future.

There are many dangers to this kind of cart-before-horse jurisprudence. One problem with customary norms, as opposed to treaty obligations, is that they are so poorly defined. Is the norm here that unilateral military action is allowed to avert a humanitarian crisis caused by the use of chemical weapons, since the use of such weapons against a government’s own people is itself a breach of international law? If so, does that mean the Syrian government is allowed to produce a humanitarian crisis as long as it does so using only conventional weapons? (This rather odd principle seems to be asserted by the British government memo referenced above.)

In any event, the Obama Administration has not yet put forth any formal argument for why it would not be violating international law by attacking Syria. If it wants to base such a claim on the enforcement of an “evolving norm” of customary international law, it should do so explicitly. To its credit, the Administration went to considerable lengths to make the 2011 bombing of Libya legal by getting approval from the U.N. beforehand. An attack on Syria now would be correctly viewed by the rest of the world as yet another depressing precedent for the principle that great powers pay attention to international law only when it’s convenient for them to do so.